Left Alone Alarm

Would like to have an alarm on a car that would go off if a child or pet is left in the car. It would need to be different than a theft sound, maybe a whistle, or even a scream. I know it takes time to load a car, so it would have to perhaps 3 to 5 minutes after the car has been shut off and locked up. The alarm would sound if around 10 pounds or more are left. Or in the case of a pet, movement in the car. Perhaps even with the alarm, the window would automatically open.
Living in Florida, I hear of babies and pets being left and they die from the heat too often. I realize it is not just in Florida either.
But just think if Ford was the first one to have this. If a car can vibrate or see behind to prevent hitting things backing up, then I think this is a reasonable idea that could be done.